Allow to ignore band with FeatureExtractor

[hombit]

Currently, FeatureExtractor analysis function requires band_to_calc parameter to specify passband it would pre-filter light curve on. This PR introduces band_to_calc=None, which tells the function to ignore passband information and apply the function to the whole light curve.

This option could be useful to some types of analysis, for example light_curve.OtsuSplit is not so sensitive to passband if it is applied to differential fluxes.

Another use case is datasets where each object is observed in a single passband, for example ZTF DRs. In such cases it could be practically useful to ignore passband information.

This PR is related to #317, but covers feature extraction only. FeatureExtractor could should be revisited again if/when #317 is implemented.

Fixes #323

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Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (4db6f6a) 94.56% compared to head (0c3e069) 94.56%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #325   +/-   ##
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  Coverage   94.56%   94.56%           
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  Files          24       24           
  Lines        1509     1510    +1     
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+ Hits         1427     1428    +1     
  Misses         82       82           

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[dougbrn]

looks good!